ruby1.9.1 on Debian isn't ruby 1.9.1. The Ruby language changed between 1.9 and 1.9.1, so a new package name had to be created. If 1.9.1 was just called "1.9"

ruby1.9.1 on Debian isn't ruby 1.9.1. The Ruby language changed between 1.9 and 1.9.1, so a new package name had to be created. If 1.9.1 was just called "1.9" it would break all of the packages in Debian that depend on whatever language features were different between 1.9 and 1.9.1. "ruby1.9.1" in Debian 7.0 provides version 1.9.3.194.